Acoustic Vector Probes
Recently a patent application was filed for a new acoustic instrument, the acoustic vector probe (AVP).                1. R. Hickling 2003, “Acoustic Measurement Method and Apparatus”, Patent Application to the United States Patent and Trademark Office, Ser. No. 10/396,541, Filing Date Mar. 25, 2003.The technical information contained in this application is hereby incorporated herein by reference.        
An AVP consists of a tetrahedral arrangement of four small microphones less than 6 mm in size that simultaneously measures at a point in air the three fundamental quantities of acoustics, namely the sound-intensity and sound-velocity vectors, and sound pressure. Sound intensity is the time average of sound power flow per unit area. The time dependence of sound intensity is determined by taking a series of averages over short intervals. AVPs are more accurate, more compact and less expensive than previous instruments for measuring sound intensity. Nested AVPs can be used to make accurate measurements over a broader frequency range than previous instruments. A calibration procedure described by Hickling (Ref. 1) ensures the probe is accurate and omnidirectional.
The sound-intensity vector determines the direction of a sound source. Because it is expressed as a fast Fourier transform (FFT), it also provides information about the frequency characteristics of the source, enabling the AVP to distinguish one source from another. Sources can also be distinguished by how they occur in time.
Arrays of Acoustic Vector Probes
Subsequently a continuation-in-part was submitted describing the use of an array of AVPs to detect buried objects                2. R. Hickling, 2003, “Method and Apparatus for Acoustic Detection of Buried Objects”, Patent Application to the United States Patent and Trademark Office, Ser. No. 10/658,076, Filing Date Sep. 9, 2003.The technical information contained in this application is hereby incorporated herein by reference. It describes how the compactness and inexpensiveness of AVPs make them suitable for forming an array. It also indicates that modern digital signal processing permits simultaneous measurement at all the AVPs.Previous Methods of Sound Source Location Using Arrays        
Previous methods of locating and quantifying sound sources using arrays have been described recently by                3. M. Batel, M. Marroquin, J. Hald, J. J. Christensen, A. P. Schuhmacher and T. G. Nielsen, 2003, “Noise Source Location Techniques—Simple to Advanced Applications”, Sound & Vibration, March issue, 24–38.These can be summarized briefly as follows.Measurements at the Source:(a) Sound pressure mapping This method consists of sound pressure measurements at different locations on the surface of a source. The method is unsatisfactory because pressure measurements do not measure sound power flow at the surface.(b) Sound intensity and selective intensity In this method a two-microphone probe is used to measure the component of sound intensity at a point perpendicular to the surface of a source. It can therefore be used to rank the sound power outputs of different components of the source and to sum these outputs to obtain the total radiated sound power. The method is quite effective. However the measurements usually have to be made by hand, and it is not easy to convince technicians to stand next to a sound source, such as an engine, for extended periods and perform careful, tedious measurements. There are also safety factors to consider. Another disadvantage is the clumsy face-to-face microphone arrangement with U-shaped holder that is used as a two-microphone probe. Because of these difficulties, there is a need to make measurements remote from the source, using methods where there is less emphasis on manual work and more on improved measurement techniques and computation.Measurements with Arrays Remote from the Source:(c) Near-field acoustic holography This method measures sound pressure at an array of individual microphones remote from the source and computes the sound field from this data. The computed field is then used to determine how the source radiates sound. However the computations can be difficult to understand and involve assumptions and approximations that can introduce misrepresentations and inaccuracies. Measurements of sound pressure in parallel planes are used to determine the components of sound velocity and sound intensity perpendicular to the planes.(d) Non-stationary acoustic holography This is a development of near-field acoustic holography for a non steady source.(e) Beam forming This method uses a phased array of individual sound-pressure microphones to form a beam with directional sensitivity, which can scan the surface of a source to obtain the approximate relative contributions of different parts of the source. Beam forming is a well-known and easily understood technique. Side lobes of the primary beam can cause error but methods developed by Batel et al can reduce this effect. A major disadvantage of the method is that it does not quantify the sound radiated by the source within the beam.(f) Inverse boundary element methods. These provide additional mathematical modeling of sound radiated by the source.Triangulation and Other Positioning and Locating Techniques        
A mathematical technique for locating sound sources using AVPs was published previously by                4. R. Hickling and A. P Morgan, 1996, “Locating sound sources with vector sound-intensity probes using polynomial continuation”, Journ. Acoust. Soc. Amer 100(1), 49–56.This method is incapable of dealing with measurement error and the finite size of sources. Hence there is a need for a least-squares triangulation formula. Triangulation is a well-known concept. Positioning systems that use triangulation have been described in texts such as        5. M. S. Grewal, A. P. Andrews and L. R. Weill, 2001, “Global Positioning Systems, Inertial Navigation and Integration”, John Wiley & Sons Inc.        6. Loran-C User Handbook, 1990, Department of Transportation, US Coast Guard, Commandant Instruction M16562.3 Washington D.C.These systems are based on time of arrival of radio waves and not on sound. Generally they consist of several transmitters and one receiver, whereas source location with an array of AVPs involves a single transmitter and a number of receivers.Arrays of Sound-Intensity Probes for Measuring Sound Power        
There are standard procedures for measuring the sound power of a source using an array of sound-intensity probes surrounding the source:                7. ISO 9614-1: 1993 (E), “Acoustics—Determination of Sound Power Levels of Noise Sources using Sound Intensity, Part I Measurement at Discrete Points”, International Organization for Standardization, Geneva, Switzerland.        8. ANSI S12-12-1992., “Engineering Methods for Determination of Sound Power Levels using Sound Intensity”, American National Standards Institute, New York.In these procedures it is assumed that two-microphone sound-intensity probes are used, aligned perpendicularly to the array. Generally such probes are clumsy and expensive and it is difficult to use them in sufficient numbers to make simultaneous measurements at all points in the surrounding array.        
In another paper the sound power of a moving source in water was determined using a single four-hydrophone vector probe.                9. W. Wei and R. Hickling, 1995, “Measuring the Sound Power of a Moving Source”, Journ. Acoust. Soc. Amer., 97(1), 116–120.Here it was assumed that the source moves along a known straight path and that its sound power can be determined by integrating over an imaginary infinite cylinder enclosing the source along its path. The four-hydrophone probe is clumsy and less compact, and is not as accurate and versatile as an AVP.        